Technics SA-700

Howdy — I have a Technics SA-700 that I've been using as a pre-amp for several years. I was simply running new ungraded interconnects between the pre and the B&K amp, only now the SA powers up but the relay never clicks and there is no sound or signal, either from the unit as a pre or as a receiver (with jumpers installed). No sound to headphones. No blown fuses, inside or out. But, as a I said, the relay never clicks. All front panel lights and indicators work, the unit just won't produce any sound. Any thoughts, or diagnosis suggestions? I have a scope, a multi-meter, and the schematics. But with the unit completely dead and no blown fuses, I'm not sure what to check next. Thanks.

If speaker relay isn't clicking I would check dc offset at the relay. It's possible there's now to much dc at the speakers. If dc offset is ok then see if you have power to the relay coil.

Edit: I'm not sure if that would affect the pre amp section though. Someone with more knowledge will chime in.

No sound from from preamp and no relay click. I think one thing I would do is check all fuses with an ohm meter if you haven't done so.

You could try to trace the signal (aux or tuner) and see where it stops. Also I would check the voltages as indicated in the service manual, especially those in the power supply.

I can't imagine how both issues could be related, unless there is a blown transistor in the pre-amp section, but then still it is quite unlikely that this happens at both channels at the same time.

In many relay protected amps, what drives the relay also drives a muting circuit somewhere between the preamp and power amp.

If it is muting the preamp I would bet on either a hard fault in the protection or an actul outut stage fault causing high DC. If it was just power to the relay itself or the relay driver transistor it would likely still have preamp output.

Okay guys, first, thanks for getting back to me so quickly. However, and unfortunately, I'm disabled and sometimes have to make unexpected trips to the hospital for some tweaking, which is why I haven't responded to your suggestions until now. But I checked all the fuses with a meter and tried to check the relay, but I'm not certain I was checking the correct pins on the board. The schematic doesn't make it clear to me which two are for the coil. But if I checked the correct ones, they are dead. But I'm still mystified as to why both the amp and preamp functions aren't working. Seems to me the the unit should still function as a pre. But the relay still isn't clicking, even with a slight tap and a quick cleaning.

if the power amp is in protection it could be muting the preamp

Well, it's in protection. The light is off on the panel. I guess it's time to check all the voltages.

I have done several Technics amp and receiver rebuilds and I own 2 pieces of SA-700 a.o., so I think I know a little on the subject.

What you are experiencing could be all kind of things, but two things are common issues on these receivers; bad solder joints and shot electrolytic capacitors. And, originating from the latter, blown transistors if you are unlucky. Then there is a specific possible issue on this SA-700, it uses 2SA798A double transistors as differential transistors and these often cause problems.

One word of warning in advance, if you are going to do repairs on it, make sure the large filter caps are bled (and the mains plug out of course) before you touch anything on the boards. As far as I remember there are no bleeding resistors on these caps and then they will stay charged for a long time.

I just had a look at the block diagram; there are two 5 amp fuses just before the relay. If there is any significant DC present on one of those, then one or more components are shot. I would say one (or both) of the 2SA798A's would be the most likely, but it just might be something else. If it were my receiver, I'd give it a recap while I'd be working on it .

Jeromach — Yeah, I'm going to do a day's worth of diagnostics on it today. I'm hopeful I can locate the problem. Unfortunately, I have to go in for another tuneup tomorrow. So the rest of the project will have to wait a week. But thanks very much for the help. I'll let you know what I find today.

You're welcome, I'm curious what you'll find.

I today was in my storage and took out one of the SA-700's I have and brought it home. I don't know if it works, but I think not (it looks shattered). I will give it a test one of these days, especially to see if the 2SA798's are bad on this one too.

Okay, I'm back to the land of the living, and did some testing today. As I said, all the fuses tested good with a multimeter, but I'm getting 40 volts at F3. I just ordered new 1.6 amp fuses, because it's the easiest thing to check. I'll replace the fuse and see what I get. It seems unlikely that the fuse would test as good if it isn't. But I've seen it happen before. And a fuse is the only thing that makes sense, given what I was doing when the unit quit. I'll let you know what I find. The fuses will be here in a couple days. However, on the schematic, I see F601 and F602, the 5 amp fuses at the relay, but I can't find them anywhere on the actual board. Any advice.

scottonnob said:

I see F601 and F602, the 5 amp fuses at the relay, but I can't find them anywhere on the actual board. Any advice.

Click to expand...

I think those would be the two speaker fuses on the rear panel of the receiver.

It occurred to me that they might be the speaker fuses, but the schematic shows them as before the relay, which is where I want to measure the DC offset. I could be wrong, but the other thing is, the speaker fuses are 6 amp, not 5, as the schematic indicates the relay fuses should be.

So here is what I see.
If the protect is on, and no click of the relay, and no sound from speakers or from the pre out's, I would think you have a regulator problem.
The unit has a +30v, a -30V regulator Circuit.
The thing I see in common is that the +30vdc regulator feeds the power amps, it feeds the Tone amps, the Equalizer, and the protect relay and circuit.
If this 30vdc regulator is bad, your display in front will work, not sure if tuner will work, but both power amps wont work, and you will not get any output on the Preamp outs either.
I would check the emitter of TR701 (or check both sides of R707, and R708, for around 30Vdc if you cant get to the emitter of the regulator.) and look for the necessary +30Vdc. While you are at it check the emitter of TR702 for -30Vdc (or both sides of R709). If the +30vdc is missing on TR701, check the collector of the transistor (metal heat sink) for approx +55Vdc. (and -55Vdc on collector of TR702). If you have no voltage on the emitter, and 55Vdc or more on the collector, then the transistor is probably open. But make sure that the solder connections for these regulators are OK, as they run very hot and can create very bad solder connections or even burn open the foil trace. And while looking at the connections on the board, make sure you don't have any cracks on the board, that can show up usually where the board mounts to the chassis, or where something heavy is mounted on the board.
Good luck